Bleach activators are understood to be compounds that react with hydrogen peroxide or perhydrates in aqueous solution to form peracids that act as a bleach. Compounds that are especially effective as bleach activators are N-acylated amines, amides, and glycolurils, which are disclosed in German Patent Applications DE No. 11 62 967, DE No. 12 91 317, DE No. 20 38 106, and DE No. 15 94 865. German Patent Application DE No. 11 62 967 suggests that these bleach activators should be coated with a water soluble coating prior to use in detergents and bleaching agents; and that the coating material can consist of carboxymethyl cellulose. This coating material can be dissolved in water in a finely dispersed form, and sprayed onto the activator, followed by drying of the coated activator. This patent recommends granulating the activator prior to coating; however, no instructions are given as to how to granulate or what granulating devices should be used. If one follows the outline of DE No. 11 62 967, and sprays such a bleach activator, e.g. tetraacetylethylenediamine, with an aqueous carboxymethyl cellulose solution in a granulator, problems will arise. The reason for this is that aqueous solutions containing more than 5 wt. % of carboxylmethyl cellulose cannot be handled properly by commercial granulation processes due to their high viscosity and gelatinous consistency. In order to be able to apply a sufficiently heavy coating on the activator particles, large amounts of dilute cellulose ether solutions must be used. As disclosed in Example 10 of German Patent Application DE No. 11 62 967, 18 wt. % carboxymethyl cellulose must be applied onto the bleach activator. When taking into consideration the fact that due to its high viscosity, a 5% solution thereof is just barely usable, the use of 18 wt. % carboxymethyl cellulose means that 360 wt. % (in relation to the amount of activator) of a 5% cellulose ether solution is required. It can be demonstrated that when using more than 20 to 30 Wt. % of such solution, the resulting mixture turns into a lumpy to paste-like material instead of useable granulates. It is therefore suggested in column 3 of German Patent Application DE No. 11 62 967 to use alcohol solutions of the carboxymethyl cellulose. However, this requires the installation of expensive and explosion proof equipment; and in addition, high costs ensue when reclaiming the solvent. Such a process, therefore, cannot be used for commercial purposes. Identical problems arise if instead of a cellulose ether solution, fatty acids, fatty acid alkanolamides, fatty alcohols, and carbowaxes dissolved in organic solvents are used as coating materials, as is also suggested in DE No. 11 62 967. An additional disadvantage is also that such coating materials do not dissolve, or dissolve only very slowly, in bleaching solutions and, therefore, the desired cold bleach effect is suppressed.
As is disclosed in German Patent Application DE No. 20 48 331, a process for the manufacture of coated, granular bleach activators is known where the activator is mixed dry with materials suitable for coating or granulating. During the second step of this process, spraying and granulating is achieved with granulating adjuvants or film formers dissolved in water. In order to obtain dry pre-mixes, it is suggested that either water soluble builder salts used in detergents that bind water as water of crystallization, e.g. phosphates, polyphosphates, carbonates, and silicates of alkali metals, or water insoluble fillers such as silicic acid, magnesium silicate or magnesium oxide be used. The same water soluble salts that bind water as water of crystallization can also be used for granulating adjuvants; or if the dry premixes are sprayed with an aqueous solution of coating forming materials such as cellulose derivatives or other water soluble polymers of natural or synthetic origin, they will granulate at the same time. This process, however, can only be used for granules with a low content of bleach activators, i.e. it is only useable for granules containing less than 50 wt. % bleach activators. The granules, therefore, are only suitable for uses where a high amount of admixturing agents will not interfere with their efficiency.
The prior art processes for coating bleach activators are all restricted to granulation processes. The usual granulation processes, however, suffer from non-uniform grain sizes. After the granulation and drying processes are completed, coarse grains and dust must be eliminated by a screening process. If the coarse grains are then ground to the desired grain size, and then added to the finished granulate, the ground mixture would have a short shelf life because the bleach activator granules will either not be protected at all or will be insufficiently protected in locii where fracturing had occurred. This means that one has to process by granulating not only the finished product itself but also the ground coarse granules, requiring the recycling of a considerable quantity of materials.
From German Patent Applications DE No. 21 38 584 and DE No. 22 07 974, methods for the manufacture of coated bleach activators are known; e.g., melted mixtures of polyglycol and fatty acids containing dispersed bleach activators are sprayed in a spray tower by means of nozzles or rotating disks, and the formed droplets are solidified by cool air. The bleach activator is very well protected against decomposition in the prills that result from this process. However, problems can occur during application because these prills will only completely dissolve above their melting point. Additions of fatty alcohols or their ethoxylates and sulfates, as described in German Patent Applications DE No. 23 38 412 and DE No. 25 35 731, only produce partial improvement of this solubility problem. Prills not dissolved at low temperatures can adhere to textiles during the laundry process, and can dissolve into small greasy spots during subsequent hot air drying or ironing. Finally, this process also causes problems during the hot summer days if insufficiently cool air is available for cooling and solidifying the prills. Pre-cooling of the air used in the spray tower is rather expensive.
The above mentioned granulating and spray processes have in common that they must be carried out at relatively low temperatures. Due to the known thermal sensitivity of the bleach activators, it was assumed until now that the bleach activators cannot be exposed to high temperature spray drying.